High dose combination chemotherapy or chemoradiotherapy followed by infusion of autologous hematopoietic stem cells is being increasingly used in the treatment of cancer. Currently, the overwhelming cause of treatment failure after autologous transplantation is relapse of malignant disease. The development of immunotherapies that can induce an autologous graft-versus-tumor (GVT) effect might significantly reduce the risk of recurrent cancer and thus improve the outcome of autologous transplantation. Substantial preclinical data support the use of IL-12 as an immunotherapeutic agent for cancer. The proposed study is a phase I clinical trial of recombinant human IL-12 after autologous hematopoietic stem cell transplantation for malignant disease. The primary objective is to assess the toxicity and maximum tolerated dose of IL-12 administered intravenously in the proposed schedule. Secondary objectives are to assess the hematopoietic and immunologic effects of IL-12 therapy. Pharmacodynamic studies will be undertaken to assess the effects of IL-12 on leukocyte counts, lymphocyte subsets, and lymphocyte function. Induction of secondary cytokines in vivo during IL-12 treatment will also be investigated.